1. Field of the Invention
The present invention relates generally to an air/fuel ratio feedback control system for an internal combustion engine. More specifically, the invention relates to an air/fuel ratio feedback control system for performing .lambda. control of feedback control in order to maintain the air/fuel ratio close to a stoichiometric value.
2. Description of The Background Art
In recent years, various types of fuel injection control systems for internal combustion engines, which can perform air/fuel ratio feedback control for an air/fuel mixture to be introduced into the engine combustion chamber, have been proposed. One such system is disclosed in Japanese Patent First (unexamined) Publication (Tokkai Sho.) No. 60-240840. The disclosed system detects an intake air flow rate Q, an intake pressure PB and so forth as quantities measuring the state of intake air, to derive a basic fuel injection amount Tp on the basis of these quantities and an engine revolution speed N. The basic fuel injection amount Tp is modified to derive a fuel injection amount Ti by utilizing various correction coefficients COEF, LAMBDA and Ts in accordance with the following formula. EQU Ti=Tp.times.COEF.times.LAMBDA+Ts
The COEF is a combined correction coefficient derived on the basis of various kinds of engine running states, such as an engine coolant temperature and so forth. The LAMBDA is an air/fuel ratio feedback correction coefficient set on the basis of an air/fuel ratio of an air/fuel mixture, which is derived from oxygen concentration contained in exhaust gas. The correction coefficient Ts is a correction value for compensating for battery voltage. Fuel, of an amount corresponding to the modified fuel injection amount Ti, is introduced into the engine combustion chamber by means of an electromagnetic fuel injection valve and so forth.
The air/fuel ratio feedback correction coefficient LAMBDA is generally derived through a PI (proportional-integral) control process. The correction coefficient LAMBDA consists of a rich control proportional component P.sub.R which is used when the air/fuel ratio varies from rich to lean across a stoichiometric value, a lean control proportional component P.sub.L which is used when the air/fuel ratio varies from lean to rich across the stoichiometric value, a rich control integral component I.sub.R which is used while the air/fuel mixture is held lean, and a lean control integral component I.sub.L which is used while the air/fuel mixture is held rich. The integral components are derived by integrating an integral constant over a period while the air/fuel mixture is maintained rich or lean. In the practical process, the correction coefficient LAMBDA is derived on the basis of the deviation of the air/fuel ratio from the stoichiometric value. The air/fuel ratio is derived on the basis of the means of an oxygen (O.sub.2) sensor.
When the air/fuel ratio is richer than the stoichiometric value, the correction coefficient LAMBDA is decreased by the lean control proportional component P.sub.L, and then it is gradually decreased in accordance with the lean control integral component I.sub.L so as to prevent the air/fuel ratio from rapidly decreasing. Thereafter, when the air/fuel ratio varies from rich to lean across the stoichiometric value, the correction coefficient LAMBDA is increased by the rich control proportional component P.sub.R, and then it is gradually increased in accordance with the rich control integral component I.sub.R so as to prevent the air/fuel ratio from being rapidly increased. Such processes are repeatedly performed, to cause the air/fuel ratio to approach the stoichiometric value.
As the oxygen sensors for the air/fuel ratio feedback control, sensors can be generally used which detect whether the air/fuel ratio is held rich or lean relative to the stoichiometric value by utilizing the fact in that the oxygen concentration in the exhaust gas rapidly varies at the stoichiometric value. One of such sensors is disclosed in Japanese Utility-Model First (unexamined) Publication (Jikkai Sho.) No. 63-51273. The disclosed sensor is formed with electrodes on inner and outer surfaces of a zirconia tube, and it monitors electromotive force produced between the electrodes in accordance with a ratio of oxygen concentration in the atmospheric air introduced into the interior of the tube to that in the exhaust gas to which the outer surface of the tube is exposed, to indirectly detect whether the air/fuel ratio for the air/fuel mixture introduced into the engine is held rich or lean relative to the stoichiometric value.
In a case where such an oxygen sensor is used for the air/fuel ratio feedback control, if the oxygen sensor deteriorates. The output characteristic of the detection signal relative to the air/fuel ratio varies from the output characteristic of the initial oxygen sensor when it is initially used, as shown in FIGS. 1 to 4, so that the air/fuel ratio can not be controlled to approach near the stoichiometric value by the feedback control.
Some exhaust systems for automotive engines are provided with a catalytic converter rhodium (CCRO) system which converts harmful gas components, such as carbon monoxide (CO), hydrocarbon (HC) and nitrogen oxides (NOx), in the exhaust gas into harmless components, such as carbon dioxide (CO.sub.2), aqueous vapor (H.sub.2 O) and nitrogen (N.sub.2) to purify the exhaust gas. Since conversion efficiency by the catalytic converter rhodium system is best when the air/fuel mixture which is burned is such that the air/fuel ratio is the stoichiometric value, if the air/fuel ratio controlled by the feedback control deviates from the stoichiometric valve due to deterioration of the oxygen sensor, there is a disadvantage in that the conversion efficiency by the catalytic converter rhodium system decreases and concentrations of harmful components, such as CO, HC and NOx, in the exhaust gas increase.
Even if there is little variation in the static characteristic of the oxygen sensor, if, for example, response time of the oxygen sensor when the air/fuel ratio varies from rich to lean or lean to rich across a stoichiometric value when the sensor is no longer new, varies from a response time when the oxygen sensor was new, there is also a disadvantage in that the set point of the air/fuel ratio deviates from the initial set point (the stoichiometric value) so that the exhaust gas can not be sufficiently purified by means of the catalytic converter rhodium system.
Variations in output characteristics of oxygen sensors due to deterioration thereof, as shown in FIGS. 1 to 4, are described below, respectively.
FIG. 1 shows a relationship between the output voltage of an oxygen sensor and the air/fuel ratio for the air/fuel mixture in a case where, for example, a well-known zirconia tube type oxygen sensor is used in a condition where a small amount of heat deterioration has occurred in the zirconia, compared to a similar oxygen sensor when new. In this case, the output characteristic of the used oxygen sensor shifts in a rich direction from the characteristic of the new oxygen sensor. In addition, as shown in FIG. 5 and the Table below, the response time of the used oxygen sensor when the air/fuel ratio varies from rich to lean across the stoichiometric value, becomes shorter than the initial response time, i.e. the response time when the sensor is new, so that the control frequency becomes higher than the initial frequency. Therefore, when feedback control is performed by using such a deteriorated (used) oxygen sensor, the air/fuel ratio is so controlled as to approach a richer value than the stoichiometric value.
TABLE ______________________________________ RE- CON- SPONSE A/F TROL BAL- RATIO OUTPUT FRE- ANCE SET Rich Lean QUENCY (FIG. 5) POINT ______________________________________ HEAT -- -- HIGH A, b RICH DETERIO- RATION SMALL INSIDE LOW LOW -- A, a RICH DETERIO- RATION OUTSIDE -- HIGH LOW A LEAN BLINDING c or d HEAT LOW -- LOW B or C RICH DETERIO- a RATION LARGE ______________________________________
In addition, as shown in FIG. 2, when such heat deterioration becomes great (for example, after the sensor has seen considerable use), the output (the maximum voltage) while the air/fuel mixture is held rich, is decreased, as a result, the control frequency of the oxygen sensor becomes lower than the initial control frequency, and the response speed becomes low and a normal output characteristic of oxygen sensor, such that the output thereof rapidly varies at the stoichiometric value of the air/fuel ratio, cannot be obtained.
As mentioned above, in a case where a zirconia tube type oxygen sensor is used, atmospheric air is introduced into the interior of the zirconia tube, and an electromotive force is produced between the electrodes formed on inner and outer surfaces in accordance with a ratio of the oxygen concentration in the atmosphere to that in the exhaust gas. Therefore, if the electrode formed on the inner surface deteriorates or if blinding is produced in a layer which inhibits the zirconia tube from directly sensing the exhaust gas, the output characteristic of the oxygen sensor varies as shown in FIGS. 3 and 4.
That is, when the inner electrode deteriorates, outputs of the oxygen sensor on both of the rich and lean sides (the maximum and minimum output voltages) are decreased since the electromotive force can be not sufficiently sensed. As a result, the set point to which the air/fuel ratio is controlled to approach by the feedback control, moves toward a richer value than the stoichiometric value. On the other hand, when blinding is produced on the outer protective layer, the output of the oxygen sensor on the lean side (the minimum output voltage) becomes high, since the ratio of oxygen concentration in the exhaust gas outside of the tube to that in the atmospheric air introduced into the tube can not increase while the air/fuel ratio is held lean. As a result, the response characteristic of the oxygen sensor when the air/fuel ratio varies from rich to lean across the stoichiometric value, becomes poor, so that the set point of the air/fuel ratio moves toward a leaner value than the stoichiometric value.